Figure REF (b) shows the measured SH power and the normalized SH efficiency as a function of the pump wavelength. Based on the calibrated insertion loss of 15 dB/facet, the experimental peak efficiency reaches 2900\(\pm \) 400%/W/cm\(^2\)  which is lower than the theoretical efficiency by \(2.5\,\)  times as indicated in the black curve. The most likely reason for the lower than the theoretical efficiency and also the undesired side-lobes is the inhomogeneity of the film thickness, which falls within the range of a few nm [1]}, [2]}, [3]}. The thickness variation causes the phase-matched poling period to vary along the length of the PPLN waveguide thus broadening the output spectrum. Experimentally, we observed the SH peak to vary \(\sim 0.21\,\) nm per nm change of the poling period. From simulation we estimate the poling period to vary \(\sim 2.25\,\) nm per nm variation of the total thickness. So 1 nm variation of the film thickness along the PPLN waveguide can shift the SHG spectrum by \(\sim 0.5\,\) nm thus drastically effecting the SHG spectrum and the SHG efficiency. The impact of film thickness variation is more prominent at deep-visible short wavelengths because of strong dispersion in this regime, which leads to large variations of poling period. The domain size variation along the waveguide due to poling non-uniformity may also influence the SH conversion efficiency due to the much shorter poling period utilized here.
